As technologies evolve, complementary metal-oxide semiconductor (CMOS) image sensors are gaining in popularity over traditional charged-coupled devices (CCDs) due to certain advantages inherent in the CMOS image sensors. In particular, a CMOS image sensor may have a high image acquisition rate, a lower operating voltage, lower power consumption and higher noise immunity. In addition, CMOS image sensors may be fabricated on the same high volume wafer processing lines as logic and memory devices. As a result, a CMOS image chip may comprise both image sensors and any necessary logic, such as amplifiers, A/D converters, or the like.
CMOS image sensors generally are pixelated metal oxide semiconductors. A CMOS image sensor typically comprises an array of light sensitive picture elements (pixels), each of which may include transistors, capacitors, and a photo-sensitive element. A CMOS image sensor utilizes light-sensitive CMOS circuitry to convert photons into electrons. The light-sensitive CMOS circuitry typically comprises a photo diode formed in a substrate. As the photo diode is exposed to light, electrical charges are induced in the photo diode. Each pixel may generate electrons proportional to the amount of light that falls on the pixel when light is incident on the pixel from a subject scene. Furthermore, the electrons are converted into a voltage signal in the pixel and further transformed into a digital signal by means of an A/D converter. A plurality of periphery circuits may receive the digital signals and process them to display an image of the subject scene.
A CMOS image sensor may comprise a plurality of additional layers, such as dielectric layers and interconnect metal layers, formed on top of the substrate, wherein the interconnect layers are used to couple the photo diode with peripheral circuitry. The side having additional layers of the CMOS image sensor is commonly referred to as a front side, while the side having the substrate is referred to as a backside. Depending on the light path difference, CMOS image sensors can be further divided into two major categories, namely front side illuminated (FSI) image sensors and backside illuminated (BSI) image sensors.